3-hydroxypropionic acid (3-HP) is a three carbon carboxylic acid identified by the U.S. Department of Energy as one of the top 12 high-potential building block chemicals that can be made by fermentation. Alternative names for 3-HP, which is an isomer of lactic (2-hydroxypropionic) acid, include ethylene lactic acid and 3-hydroxypropionate. 3-HP is an attractive renewable platform chemical, with 100% theoretical yield from glucose, multiple functional groups that allow it to participate in a variety of chemical reactions, and low toxicity. 3-HP can be used as a substrate to form several commodity chemicals, such as 1,3-propanediol, malonic acid, acrylamide, and acrylic acid. Acrylic acid is a large-volume chemical (>7 billion lbs/year) used to make acrylate esters and superabsorbent polymers, and is currently derived from catalytic oxidation of propylene. Fermentative production of 3-HP would provide a sustainable alternative to petrochemicals as the feedstock for these commercially-significant chemicals, thus reducing energy consumption, US dependence on foreign oil, and the production of greenhouse gases.
Bacteria can be used to ferment sugars to organic acids. However, bacteria present certain drawbacks for large-scale organic acid production. As organic acids are produced, the fermentation medium becomes increasingly acidic. Lower pH conditions are actually preferable, because the resultant product is partially or wholly in the acid form. However, most bacteria that produce organic acids do not perform well in strongly acidic environments, and therefore either die or begin producing so slowly that they become economically unviable as the medium becomes more acidic. To prevent this, it becomes necessary to buffer the medium to maintain a higher pH. However, this makes recovery of the organic acid product more difficult and expensive.
There has been increasing interest in recent years around the use of yeast to ferment sugars to organic acids. Yeasts are used as biocatalysts in a number of industrial fermentations, and present several advantages over bacteria. While many bacteria are unable to synthesize certain amino acids or proteins that they need to grow and metabolize sugars efficiently, most yeast species can synthesize their necessary amino acids or proteins from inorganic nitrogen compounds. Yeasts are also not susceptible to bacteriophage infection, which can lead to loss of productivity or of whole fermentation runs in bacteria.
Although yeasts are attractive candidates for organic acid production, they present several difficulties. First, pathway engineering in yeast is typically more difficult than in bacteria. Enzymes in yeast are compartmentalized in the cytoplasm, mitochondria, or peroxisomes, whereas in bacteria they are pooled in the cytoplasm. This means that targeting signals may need to be removed to ensure that all the enzymes of the biosynthetic pathway co-exist in the same compartment within a single cell. Control of transport of pathway intermediates between the compartments may also be necessary to maximize carbon flow to the desired product. Second, not all yeast species meet the necessary criteria for economic fermentation on a large scale. In fact, only a small percentage of yeasts possess the combination of sufficiently high volumetric and specific sugar utilization with the ability to grow robustly under low pH conditions. The U.S. Department of Energy has estimated that production rates of approximately 2.5 g/L/hour are necessary for economic fermentations of several organic acids, including 3-HP (http://www1.eere.energy.gov/biomass/pdfs/35523.pdf).
Although many yeast species naturally ferment hexose sugars to ethanol, few if any naturally produce significant yields of organic acids. This has led to efforts to genetically modify various yeast species to produce organic acids. Genetically modified yeast strains that produce lactic acid have been previously developed by disrupting or deleting the native pyruvate decarboxylase (PDC) gene and inserting a lactate dehydrogenase (LDH) gene to eliminate ethanol production (see, e.g., WO99/14335, WO00/71738, WO02/42471, WO03/049525, WO03/102152 and WO03/102201). This alteration diverts sugar metabolism from ethanol production to lactic acid production. The fermentation products and pathways for yeast differ from those of bacteria, and thus different engineering approaches are necessary to maximize yield. Other native products that may require elimination or reduction in order to enhance organic acid product yield or purity are glycerol, acetate, and diols. The reduction of glycerol in genetically altered yeast strains is described in, for example, WO07/106,524.
Unlike lactic acid, 3-HP is not a major end product of any pathway known in nature, being found in only trace amounts in some bacteria and fungi. Thus, a greater deal of genetic engineering is necessary to generate yeast that produce 3-HP. A Saccharomyces cerevisiae strain was previously engineered to produce 3-HP at very low levels through a lactate intermediate (see WO02/042418). However, the tolerance level of wild-type S. cerevisiae is insufficient to make it an optimal host for 3-HP production. Therefore, there is a need for improved yeast strains that generate 3-HP in a more cost-effective manner on an industrial scale.